Grand Canyon provenance for orthoquartzite clasts in the lower Miocene of coastal southern California

This research was supported by National Science Foundation (NSF) grants EAR 10-19896 and EAR 14-51055 awarded to B. Wernicke, EAR 17-28690 awarded to J. Stock, and OPP 13-41729 awarded to J. Kirschvink. We also acknowledge NSF grant EAR 16-49254 awarded to G. Gehrels at the University of Arizona for support of the Arizona LaserChron Center.Orthoquartzite detrital source regions in the Cordilleran interior yield clast populations with distinct spectra of paleomagnetic inclinations and detrital zircon ages that can be used to trace the provenance of gravels deposited along the western margin of the Cordilleran orogen. An inventory of characteristic remnant magnetizations (CRMs) from >700 sample cores from orthoquartzite source regions defines a low-inclination population of Neoproterozoic-Paleozoic age in the Mojave Desert-Death Valley region (and in correlative strata in Sonora, Mexico) and a moderate- to high-inclination population in the 1.1 Ga Shinumo Formation in eastern Grand Canyon. Detrital zircon ages can be used to distinguish Paleoproterozoic to mid-Mesoproterozoic (1.84-1.20 Ga) clasts derived from the central Arizona highlands region from clasts derived from younger sources that contain late Mesoproterozoic zircons (1.20-1.00 Ga). Characteristic paleomagnetic magnetizations were measured in 44 densely cemented orthoquartzite clasts, sampled from lower Miocene portions of the Sespe Formation in the Santa Monica and Santa Ana mountains and from a middle Eocene section in Simi Valley. Miocene Sespe clast inclinations define a bimodal population with modes near 15 degrees and 45 degrees. Eight samples from the steeper Miocene mode for which detrital zircon spectra were obtained all have spectra with peaks at 1.2, 1.4, and 1.7 Ga. One contains Paleozoic and Mesozoic peaks and is probably Jurassic. The remaining seven define a population of clasts with the distinctive combination of moderate to high inclination and a cosmopolitan age spectrum with abundant grains younger than 1.2 Ga. The moderate to high inclinations rule out a Mojave Desert-Death Valley or Sonoran region source population, and the cosmopolitan detrital zircon spectra rule out a central Arizona highlands source population. The Shinumo Formation, presently exposed only within a few hundred meters elevation of the bottom of eastern Grand Canyon, thus remains the only plausible, known source for the moderate- to high-inclination clast population. If so, then the Upper Granite Gorge of the eastern Grand Canyon had been eroded to within a few hundred meters of its current depth by early Miocene time (ca. 20 Ma). Such an unroofing event in the eastern Grand Canyon region is independently confirmed by (U-Th)/He thermochronology. Inclusion of the eastern Grand Canyon region in the Sespe drainage system is also independently supported by detrital zircon age spectra of Sespe sandstones. Collectively, these data define a mid-Tertiary, SW-flowing "Arizona River" drainage system between the rapidly eroding eastern Grand Canyon region and coastal California.Publisher PDFPeer reviewe

Volcanic and Tectonic Constraints on the Evolution of Venus

Surface geologic features form a detailed record of Venus’ evolution. Venus displays a profusion of volcanic and tectonics features, including both familiar and exotic forms. One challenge to assessing the role of these features in Venus’ evolution is that there are too few impact craters to permit age dates for specific features or regions. Similarly, without surface water, erosion is limited and cannot be used to evaluate age. These same observations indicate Venus has, on average, a very young surface (150–1000 Ma), with the most recent surface deformation and volcanism largely preserved on the surface except where covered by limited impact ejecta. In contrast, most geologic activity on Mars, the Moon, and Mercury occurred in the 1st billion years. Earth’s geologic processes are almost all a result of plate tectonics. Venus’ lacks such a network of connected, large scale plates, leaving the nature of Venus’ dominant geodynamic process up for debate. In this review article, we describe Venus’ key volcanic and tectonic features, models for their origin, and possible links to evolution. We also present current knowledge of the composition and thickness of the crust, lithospheric thickness, and heat flow given their critical role in shaping surface geology and interior evolution. Given Venus’ hot lithosphere, abundant activity and potential analogues of continents, roll-back subduction, and microplates, it may provide insights into early Earth, prior to the onset of true plate tectonics. We explore similarities and differences between Venus and the Proterozoic or Archean Earth. Finally, we describe the future measurements needed to advance our understanding of volcanism, tectonism, and the evolution of Venus

Provenance, Structural Geology, and Sedimentation of the Miocene and Pliocene Californias

The first chapter of this thesis documents a provenance study, in which orthoquartzite clasts deposited in the Miocene Sespe Formation are linked to the Mesoproterozoic Shinumo Quartzite. The Sespe Formation outcrops in the Santa Monica Mountains and the Santa Ana Mountains, both in California. The Shinumo Quartzite outcrops only in Grand Canyon. We determine that the Shinumo Quartzite can be distinguished from other sources that may feed the Sespe Formation through its unique combination of a moderate paleomagnetic inclination and 1.2, 1.4, and 1.7 Ga detrital zircon spectrum peaks. This provenance link places an important constraint on the drainage of a paleo-Colorado River from Grand Canyon during Miocene time. The second and third chapters of this thesis are hinged upon a geologic mapping project on Isla Ángel de la Guarda, a microcontinental block, in Baja California, Mexico. A plate reorganization at the end of the late Miocene andesitic arc marks the transfer of Baja California and the not-yet-rifted Isla Ángel de la Guarda to the Pacific plate from the North American plate. Between 3 and 2 Ma, the plate boundary jumped again, northward along the Ballenas Transform fault. In this Pliocene time, units mapped in this study were deposited. The oldest units mapped are Miocene-Pliocene volcanic flows, for which we have no lower age constraint. The oldest volcanic flow dated is a Pliocene andesite lava (3.916 ± 0.088 Ma from 40Ar/39Ar). We map Miocene to Pliocene volcanic flows and Pliocene to Quaternary sedimentary units in two field areas. The sedimentary units are probably results of Pliocene rifting-related basin subsidence. Geochemical data from X-ray fluorescence indicate that lavas are compositionally similar to ~12 Ma arc-related rocks mapped in the Puertecitos Volcanic Province. In the southern field area, the sedimentary units are overlain by a Pliocene basaltic andesite with an 40Ar/39Ar age of 2.754 ± 0.021 Ma. We map several NNE-striking faults throughout both field areas, which cut NNW-striking bedding in Pliocene units. The Pliocene volcanic flows and sedimentary units were probably tilted before faulting, and the faults are likely linked to the Northern Salsipuedes Basin, offshore of the island in the Ballenas Channel. Both of these events may be results of 3-2 Ma rifting.</p

Detrital zircon ages of Sespe Formation orthoquartzites

Detrital zircon ages from orthoquartzite clasts in the Sespe FormationRelated Publication: Grand Canyon provenance for orthoquartzite clasts in the lower Miocene of coastal southern California Sabbeth, Leah Caltech Geosphere en

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